I have the white back, as i picked it so fingerprints wont show. I'm from Australia and mine came with it (i9023 version), all though i cannot find any stores selling them.
But I searched eBay and only could find a white over for the i9020 from China.
I don't know if it will fit any other models.
Search on eBay:
"NEW Original White Battery Door Back Cover For Samsung Nexus S i9020 I-9020"
"Housing Back Battery Cover Door For Samsung Nexus S i9020 White"
I cant post the link as I'm a new member.